So are you placing the burden of the 'must" in 18.15 below on the ref, not placing the burden of the "must" on the non throwing team despite the law sanction of a free kick against non throwing team in 18.15?
The non-throwing team must have a player between the touchline and the five-metre line. The player stands two metres from the mark of touch on their team’s side of the lineout and two metres from the five-metre line. Sanction: Free-kick.
I contend the burden of must is on non throwing team and is not the referee obligation other than sanction. Afterall a free kick against the referee is non existent to date, so far, but may be something to look forward in future
All laws are
implicitly a must, else what's the point of any of them. The
how and
when they are applied is the part to our discretion. But to Dickie E's point, if you verbalize to the non-throwing team that they must have a player in position there, at the first, second, maybe remind them at the third lineout if you're so kind, before commencing the lineout, that's just good game management as would similarly be elsewhere when the ref communicates to prevent infraction and keep the game going.
If by the 3rd lineout or after, or if the same non-throwing team didn't react / intentionally chose not to listen to my command that they must have a player in that position, that's when I think it's fair game to ping them, to get them back on track.
There's some sort of saying about refereeing rugby: "
don't look for penalties to hand out" (I forget the exact words). The laws are less technical than most other sport's rules, e.g. American football which has a penalty every other play lol.